The present invention relates to light emitting diodes (LEDs) formed in wide bandgap materials and in particular relates to improving the extraction of light from such LEDs.
A light emitting diode is a semiconductor photonic device that emits light upon the recombination of electrons and holes in the semiconductor material or material system. The recombination is typically driven by a voltage bias across p-type and n-type materials that form a p-n junction. Because the recombination is a quantum mechanical step, the photon generated, its energy, and thus the frequency and wavelength (perceived as color) of the photon will depend upon the maximum energy of a permitted recombination transaction.
The visible colors green, blue, and violet, along with the ultraviolet portion of the electromagnetic spectrum, represent higher frequencies and thus higher energy photons. As a result, blue light can only be produced by materials with bandgaps of at least about 2.6 electron volts (eV). In turn, because blue is a primary color and is particularly desirable for full-color devices and devices that produce white light (as a combination of red, green, and blue), much recent interest has focused upon improvements in wide bandgap light emitting diodes form from materials such as silicon carbide, diamond, and the Group III nitrides. In particular, light emitting diodes formed of Group III nitride active regions continue to gain increasing commercial acceptance and are becoming more common in everyday applications.
A number of factors taken together produce the visible emission of a light emitting diode. As a potential limiting factor, however, not every recombination-generated photon externally exits the physical diode. Stated differently, a given voltage will produce a given number of recombination events which will in turn generate a given number of photons (not necessarily the same number). Not all of the generated photons, however, will be externally emitted as visible light. Instead, the photons are subject to competing factors including reabsorption and internal reflection. Accordingly, all other factors being equal, one goal for increasing the visible output of a light emitting diode is to increase the fraction (percentage, proportion) of photons that physically escape the diode in an intended illuminating direction.
The effect of Snell's law represents another factor in the external emission of an LED; i.e., the behavior of light as it meets an interface between two different materials. Specifically, when light waves reach such an interface, they will either reflect or refract. The difference (as well as any angle of refraction) depends upon the index of refraction of the adjacent material and the incident angle of the light. In an LED, one of the adjacent materials is a semiconductor and the other is the bordering environment. In some cases this is air, while in other cases it is a lens material, frequently a polymer transparent to frequencies within the visible range. Increasing the number of different angles at which emitted photons meet the boundary correspondingly increases the statistical probability that more photons will be emitted, rather than internally reflected. Accordingly, as the electronic efficiency of Group III nitride based diodes has increased, opportunities to increase efficiency based on the boundary have become more attractive. Examples of these efforts include (but are not limited to) U.S. Pat. No. 6,791,119 and U.S. Patent Application Publication Nos. 20040041164 and 20050247950 which are commonly assigned with the present invention.